1. Field of the Invention
The present invention relates generally to the synchronization of a three-phase Alternating Current (AC) power source to a three-phase power grid. Specifically, the preferred embodiments of the present invention relate to the synchronization of the voltage phase angles of static power converters to those of a three-phase power grid.
2. Description of the Related Art
As society continues to advance technologically and economically, it becomes increasingly dependent on energy such as electrical power to drive the advancement. Indeed, the shortage of electrical power may hinder such advancement and wreak havoc on society, which relies on electricity for everyday commerce and living. The California energy crisis of 2001 highlights this reliance on electricity and demonstrates the need to provide additional electrical power generators wherever and whenever increased power consumption is detected.
As known, each geographical region in countries such as the United States is supplied with electricity from power plants via a three-phase power grid. Traditionally, large power plants were built to meet the increased demand of power consumption. However, large power plants require large transmission lines to supply electricity to regional power grids, and new plants and transmission lines require substantial financial investment. Additionally, obtaining governmental approval to site and build new power plants and transmission capacity is becoming more difficult due to complex issues ranging from environmental concerns and potential health effects of electromagnetic fields (EMF) to other special interest groups' concerns.
An alternative to large power plants is the use of small low cost power generators connected in parallel to power grids to provide added power capacity during peak power consumption periods in order to reduce the strain on power grids. For instance, a peak period may occur during the summer, when widespread air conditioning use becomes taxing on the power grids. AC power sources, such as static power converters (SPCs) drawing power from batteries, fuel cells, and the like, have been used as low cost power generators to provide added power capacity to power grids during peak periods. As with the connection of a large power plant to an existing power grid, when connecting an additional AC power source, such as a static power converter, to a three-phase power grid, synchronization of the amplitudes, frequencies, and phase angles of the existing voltage on the power grid and the output voltage of the AC power source is required. Without synchronization, the discrepancies in the amplitudes, frequencies, and/or phase angles between the two parallel voltages may cause abnormal current to circulate between the grid and the additional power source and damage both systems.
Typically, when connecting a three-phase AC power source such as a SPC with a three-phase power grid, a phase-locked loop (PLL) technique is used in the AC power source to control its phase synchronization with the grid. In abnormal operating conditions, if the grid voltage is lost for a short time, e.g., 0.1 second, and a circuit switch has a bounce action, the PLL will not function properly without its feedback inputs from the power grid. Thus, under fault conditions, when the PLL loses synchronization with the grid, abnormal current such as over current and distortion current may occur. Consequently, the AC power source will not realize reliable parallel operation with the grid. And as mentioned earlier, the abnormal current may also damage both the AC power source and the power grid.